This invention relates to a device for controlling Faraday rotational angle of a light which transmits through a garnet single crystal by applying external magnetic fields from more than one (or, at least two) directions to a garnet single crystal having a Faraday effect and varying the synthesized magnetic field of the more than one directionally applied external magnetic fields. More particularly, the present invention relates to and provides a Faraday rotation angle varying device, which comprises a base film of a garnet single crystal which varies Faraday rotation angle by varying the synthesized magnetic field and, in combination, a compensating film of a garnet single crystal which has a substantially fixed Faraday rotation angle, so that a wavelength variation component of the Faraday rotation angle of the base film is reduced by means of the compensating film. The Faraday rotation angle varying device of the present invention is useful particularly for optical devices such as an optical attenuator, etc.
In the field of an optical communication system, it has been required to provide an optical attenuator for controlling light transmission (that is, a transmitted light quantity), or a polarizing scrambler for continuously and cyclically varying the direction of polarization. In these devices such as the optical attenuator and polarizing scrambler is mounted a Faraday rotation angle varying device. The Faraday rotation angle varying device is constructed to control Faraday rotation angle of a light which transmits through the garnet single crystal by applying an external magnetic field to the garnet single crystal having a Faraday effect from at least two directions and varying the synthesized or composite magnetic field.
FIGS. 1A and 1B show a typical example of an optical device employing a Faraday rotation angle varying device. FIG. 1A designates an entire structure of an optical attenuator and FIG. 1B a structural feature of the Faraday rotation angle varying device. As illustrated, a polarizer 10, a Faraday rotation angle varying device 12 and an analyzer 14 are disposed in turn. An incident light from an input fiber 16 is made into parallel lights by a collimator lens 18 and passes through, in turn, the polarizer 10, a garnet single crystal 20 of the Faraday rotation angle varying device, and the analyzer 14, and then converged by a collimator lens 22 and collected by an output fiber 24. A fixed magnetic field which is parallel with the direction of light is applied to the garnet single crystal 20 by permanent magnets 26, 28, and a variable magnetic field which is perpendicular to the direction of light by means of an electromagnet 30. By changing the synthesized magnetic field to change the magnetization direction of the garnet single crystal, the Faraday rotation angle is varied so that a quantity of light transmitted to the analyzer 14 is controlled.
More specifically, the polarizer and the analyzer are aligned such that an angle of polarized surface of the light transmitting therethrough is set to be 105 degrees and, when the magnetic field of the electromagnet is zero (0), Faraday rotation angle of the garnet single crystal becomes maximum, that is, 96 degrees. Since the angle of polarizing directions of the polarizer and the analyzer is 105 degrees, a quantity of light (a quantity of outgoing light) passing through the analyzer is reduced due to an angle of deviation, the reduction being extremely small. By contrast, as the Faraday rotation angle is reduced by applying a magnetic field to the electromagnet, the angle of deviation becomes increased to thereby increase a quantity of attenuation (that is, a quantity of outgoing light is reduced). When Faraday rotation angle is 15 degrees, the analyzer is of a so-called crossed Nicol state relative to the analyzer, so that the attenuation quantity becomes maximum.
Recently, by a new a practical application of a wavelength multiplex communication system, there has been an industrial requirement that the optical device, has less wavelength dependency. Thus, it is also required that a Faraday rotation angle varying device, as well, has less wavelength dependency.
In the example of the optical attenuator described above, when a magnetic field of the electromagnet is zero (0, there is no substantial change in a quantity of light (outgoing light quantity) passing through the analyzer even when there is more or less change in Faraday rotation angle due to a wavelength variation. When a magnetic field is applied to the electromagnet to increase a deviation of angle, it causes an increase of attenuation (that is, reduction of quantity of outgoing light), and a quantity of attenuation (dB) at that time is represented by the following formula (equation):
D=xe2x88x9210*log(10(xe2x88x92ko/10)+sin2(xcex94xcex8))
provided that:
ko: extinction ratio of garnet single crystal; and
xcex94xcex8: deviation of angle from a crossed Nicol state.
From the formula described above, it is understood that a quantity of attenuation is dependent upon the square of a sine function of the deviation angle at the position near the crossed Nicol state at which a large attenuation is obtained and extremely sensitive to the angle. In other words, in this region there is a problem that an attenuation is extensively varied by a change of Faraday rotation angle due to a wavelength variation.
With reference to Faraday rotator which uses a constant state of Faraday rotation angle, there have been many attempts and suggestions to reduce a wavelength dependency as disclosed in, for example, Japanese Patent Publication (Unexamined) No. 2-256,018 and No. 10-273,397. However, with respect to a Faraday rotation angle varying device which permits variation or adjustment of Faraday rotation angle, there is no attempt for reducing a wavelength dependency and, therefore, with reference to an optical attenuator for wavelength multiplex communication, it has been strongly required that a wavelength dependency is reduced at a region adjacent to the attenuation quantity which is particularly required in accordance with the state and condition of use.
It is, therefore, a general object of the present invention to provide a new Faraday rotation angle varying device which can reduce a wavelength dependency.
Another object of the present invention is to provide a Faraday rotation angle varying device which has less wavelength dependency and less temperature dependency.
According to the present invention, there is provided a Faraday rotation angle varying device in which an external magnetic field is applied from at least two directions to a garnet single crystal having a Faraday effect and varying a synthesized magnetic field so that Faraday rotation angle of light transmitting through the garnet single crystal is controlled, comprising:
a base film of garnet single crystal having a rotation angle varied in accordance with variation of a synthesized magnetic field, and
a compensating film of a garnet single crystal having a constant Faraday rotation angle,
wherein the base film has a wavelength coefficient sign and the compensating film has a wavelength coefficient sign different from the sign of a wavelength coefficient of the base film, so that a wavelength variation component of the Faraday rotation angle of the base film is reduced by the compensating film.
The outer magnetic fields are, in general, applied from two directions, that is, a parallel direction and a perpendicular direction relative to a light direction and, in that case, it is preferred that the magnetic field which is parallel to the light direction is a fixed magnetic field formed by the permanent magnet which has a magnetic strength for permitting the base film to be magnetically saturated, whereas the perpendicular magnetic field is a variable magnetic field applied by the electromagnet.
The outer magnetic fields are always applied to the base film from the two directions or more, whereas an adjustment is made to the compensating film relative to the outer magnetic field in accordance with magnetic characteristics of the compensating film. For example, when a magnetic anisotropy parallel to the light direction of the compensating film is large, the compensating film can be arranged, as similar as the base film, in the position where both the fixed magnetic field and the variable magnetic field are applied. By contrast, when a magnetic anisotropy parallel to the light direction of the compensating, film is small, the compensating film is located at the position where a fixed magnetic field can be applied but a variable magnetic field is difficult to be applied. The compensating film can be located at the position where no outer magnetic field is applied if the compensating film has a large magnetic anisotropy parallel to the light direction and a large coercive force so that the magnetization direction is oriented in parallel to the light direction without an outer magnetic field. In either case, however, the base film and the compensating film must be inserted between the polarizer and the analyzer in a case of an application into an optical attenuator. The number of base films and compensating films to be used can be optional and selected as desired.
A structure, in which a compensating film having a large magnetic anisotropy parallel to the light direction is used, permits an easy assembly because the base film and the compensating film are mounted together in a synthesized magnetic field and, therefore, a requirement for miniaturization is not interrupted even when the conventional permanent magnet and electromagnet are used as they are. A simplified structure of the case described above is shown in FIG. 2. A base film 46 and a compensating film 48 are mounted in a magnetic field parallel to the light direction by permanent magnets 40, 42 and a magnetic field perpendicular to the light direction by the electromagnet 44. When a magnetic field by the electromagnet 44 is zero (0, a magnetization direction of the base film 46 and the compensating film 48 is identical to the light direction (see FIG. 2A). When a magnetic field by the electromagnet 44 is applied, a magnetization direction of the base film 46 is rotated but, on the other hand, a magnetization direction of the compensating film 48 is not rotated as shown in FIG. 2B. Thus, in the present invention, a Faraday rotation angle is varied in a predetermined range by the base film 46, and a wavelength dependency is reduced by the compensating film 48.
An example of a physical property of the base film and the compensating film will be described below. This is the same as the Embodiment 1 which will be described presently.
[Base Film]
Composition: Tb1.00 Y0.65 Bi1.35 Fe4.05 Ga0.95 O12 
Faraday rotation angle when magnetic field of an electromagnet is zero: 96 degrees (that is, 32 degreesxc3x973)
Wavelength changing ratio: xe2x88x920.15%/nm
[Compensating Film]
Composition: Gd1.00 Y0.75 Bi1.25 Fe4.00 Ga1.00 O12 
Faraday rotation angle when magnetic field of an electromagnet is zero: xe2x88x9219.7 degrees
Wavelength changing ratio: +0.15%/nm
Each of the base film and the compensating film was solely inserted into the outer magnetic field and a magnetic field dependency of an electromagnet was measured to obtain experimental results as shown in FIG. 3. A wavelength used therefor was 1550 nm and the results were obtained at a fixed temperature of 25xc2x0 C. With reference to the base film, a Faraday rotation angle becomes reduced when the magnetic field of an electromagnet becomes larger, but a Faraday rotation angle of the compensating film is substantially constant. The reason for this is supposedly based upon the state that a magnetization direction of the compensating film is maintained in the direction of the light even when a magnetic field is applied by the electromagnet.
In FIGS. 4A, 4B and 4C, experimental data for measurement of a wave dependency of Faraday rotation angle are shown with respect to a case in which the base film is solely used and a case in which both the base film and the compensating film are used in combination. In FIG. 4A, when a magnetic field of an electromagnet is zero (0, the case in which the base film and the compensating film are combined shows that a wavelength dependency of Faraday rotation angle is slightly smaller than the other case in which the base film is solely used. In FIG. 4B showing the case that a magnetic field of the electromagnet is 40.1 kA/m, the combination of the base film and the compensating film shows that the wavelength dependency is remarkably reduced and, in case that the magnetic field of the electromagnet is 74.5 kA/m as shown in FIG. 4C, a wavelength dependency is made extremely small and even flat-shaped relative to a change of a wavelength.
Some reasons for the above data with respect to the wavelength dependency will reside in the following. Namely, when a magnetic field of the electromagnet is zero (0), an absolute value of Faraday rotation angle of the base film is 96 degrees whereas the value (that is, an absolute value of Faraday rotation angle) of the compensating film is 19.7 degrees, that is, about one fifth (⅕) of the value of the base film and, therefore, a wavelength variation component of the base film can only be reduced to about one fifth (⅕). By contrast, in a region that a magnetic field of the electromagnet is large, a Faraday rotation angle of the base film is reduced whereas the Faraday rotation angle of the compensating film is unchanged and, therefore, a difference in the absolute values of Faraday rotation angles becomes smaller so that a compensating effect of a wavelength variation component becomes larger. When a magnetic field of the electromagnet is 74.5 kA/m, Faraday rotation angles are substantially equivalent to each other and, therefore, a varied component of wavelength is completely cancelled, so that a wavelength dependency becomes zero (0). In this case, total Faraday rotation angle, which is the sum of Faraday rotation angles of the base film and the compensating film, becomes zero (0) as well, but since a variable width of the Faraday rotation angle is not influenced, there is no problem with the Faraday rotation angle varying device. This is quite different from a Faraday rotator of the type having a fixed Faraday rotation angle. As the Faraday rotation angle of the compensating film is substantially constant, a variable width of the Faraday rotation angle is determined by a difference between a maximum value and a minimum value, respectively, of the Faraday rotation angle of the base film. Thus, when the compensating film is added, both the maximum value and the minimum value are simply increased or decreased, with the variable width being unchanged.
In the instant description of the present invention, the description xe2x80x9cFaraday rotation angle of the compensating film is substantially constantxe2x80x9d intends to mean that the Faraday rotation angle of the compensating film can be deemed as being substantially constant relative to a variable range of Faraday rotation angle of the base film. More specifically, the variable range of a Faraday rotation angle of the compensating film relative to the variable range of a Faraday rotation angle of the base film is 5% or less, and more preferably 3% or less. As a matter of course, it is best that the variable range described above be 1% or less. Although a wavelength dependency can be reduced even when a Faraday rotation angle of the compensating film is varied, a variable range of a total Faraday rotation angle sum of those of the base film and the compensating film becomes narrower and, if it is attempted to widen the narrowed range, the base film must be formed thicker, or the number of the base films to be used must be increased, or in other alternatives, a magnetic field of the electromagnet must be increased, but these are not recommended.
Provided that a maximum value of an absolute value of a Faraday rotation angle of the base film is Fa max, and an absolute value of a Faraday rotation angle of the compensating film is Fb, an inequality Fa max greater than Fb is satisfied. Especially, when a minimum value of the absolute value of a Faraday rotation angle of the base film is Fa min, it is desired that an inequality Fa max greater than Fb greater than Fa min be obtained. This will make it possible to minimize a wavelength dependency of an optional Faraday rotation angle in the midst between Fa max and Fa min. Incidentally, the absolute value Fb of a Faraday rotation angle of the compensating film can be set to an optional value by adjusting a thickness of the compensating film.
By addition of the compensating film as described above, it is possible to reduce a wavelength dependency. Particularly, it is quite useful that a wavelength dependency in a region of a large magnetic field of an electromagnet which is required in the field of an optical attenuator can be reduced to a large extent. For this purpose, it is sufficient that an absolute value Fb of Faraday rotation angle of the compensating film be set adjacent to a minimum value Fa min of an absolute value of a Faraday rotation angle of the base film.
The base film is preferably made of the material selected from a material having a composition represented by (RBi)3(FeM)5O12. The compensating film is preferably made of the material of a composition represented by, for example, R3Fe5O12 or (RBi)3(FeM)5O12 which has a compensation temperature higher than a maximum (highest) temperature of application. Here, xe2x80x9cRxe2x80x9d represents one or more chemical element(s) selected from rare earth elements including yttrium (Y), and xe2x80x9cMxe2x80x9d represents one or more element(s) which can be substituted by iron. These films can be effectively formed by LPE (liquid phase epitaxial) method. Here, the compensation temperature referred as above represents a temperature, at and by which a magnetic moment is reversed and, in a phenomenal sense, a sign of Faraday rotation angle is reversed at a boundary point of this xe2x80x9ccompensationxe2x80x9d temperature.
In the present invention, there is provided another structure in which both a wavelength dependency and a temperature dependency are reduced by determining a displacement path of a synthesized magnetic field vector of an outer magnetic field applied to the base film. Similarly, in this case, in addition to the base film of a garnet single crystal which permits a change of a Faraday rotation angle by variation of the synthesized (composite) magnetic field, the compensating film of a garnet single crystal which has a constant Faraday rotation angle is provided. However, it should be noted that the base and compensating films are selected such that the signs of a wavelength coefficient and temperature coefficient of the base film are different from signs of a wavelength coefficient and temperature coefficient of the compensating film. Both the base film and the compensating film are formed such that they are polished on the (111) plane, and that light transmits in the  less than 111 greater than direction which is perpendicular to the (111) plane. Further, they are formed such that a displacement path of the synthesized vector of the external magnetic fields is within a fan-shaped range of a stereographic projection chart of the garnet single crystal with the (111) plane taken as the center of the chart, the fan-shaped range being surrounded by two lines connecting a point indicating the (111) plane positioned at the center of the stereographic projection chart to two positions apart 5 degrees rightward and leftward from a point indicating one of crystal planes equivalent to the (110) plane positioned on the outermost peripheral circle of the stereographic projection chart, that is, a range of the shaded portion in FIG. 5. Here, the crystal planes equivalent to the (110) plane on the outermost circumference represent (-101), (-110), (01-1), (10-1), (1-10), and (0-11) planes. (Here, in the representation for designating crystallization surfaces described above, minus indexes are used in place of lateral bars which are generally used for negative indexes.)
In the stereographic projection chart of FIG. 5, the adjacent concentric circles will represent surfaces which are different from each other by 10 angular degrees, and the adjacent lines in the radial direction will represent the surfaces which are different from each other by 10 angular degrees. Accordingly, an optional surface of the garnet single crystal can be shown as a dot in the stereographic projection chart. A magnetic field in the vertical direction on the drawing paper surface of FIG. 5 is applied to the base film by a permanent magnet, and a magnetic field is applied by the electromagnet in the radially outward direction from a center of circles in the drawing.
It would be the most preferable that a displacement path of the synthesized or composite vector of the outer magnetic field applied to the base film is a line which connects between a central (111) plane in the stereographic projection chart for centering a (111) plane of the garnet single crystal and a plane which is equivalent to the (110) plane on the outermost circle. In this respect, in view of errors in production, the location in the range of the shaded fan shape in FIG. 5 will be able to restrict deterioration of properties to a minimum as long as it is placed in the shaded, fan-shaped range.
A wavelength and temperature dependency in the Faraday rotation angle varying device is represented by the following equation:
xcex8F (xcex, T)=xcex8Fmax (xcex, T)xc3x97cosxcex1(T)
In the equation stated above, xcex8Fmax is a Faraday rotation angle by Faraday effect and has wavelength and temperature dependency. As illustrated in FIG. 6, alpha (xcex1) represents an angle between a magnetization direction of the garnet single crystal and a direction of light and it is determined dependent upon magnetic anisotropy and has a temperature dependency. From the equality described above, in order to reduce the wavelength dependency of xcex8F, it is sufficient to reduce the wavelength dependency of xcex8Fmax. By contrast, the temperature dependency of xcex8F is determined by both the temperature dependency of xcex8Fmax and the temperature dependency of alpha (xcex1) and, accordingly, for the purpose of reducing the temperature dependency of xcex8F, both must be canceled with each other or both must be determined to be closer to zero. In the structure according to the present invention, both the temperature dependency of xcex8Fmax and the temperature dependency of alpha (xcex1) must be made closer to zero (0).
In other words, a temperature dependency of xcex8Fmax can be reduced by the combination of the base film and the compensating film, the latter having a sign of temperature coefficient which is different from that of the base film. Further, the direction of an outer magnetic field which is applied to the base film is determined to be located within the range specified above, and a contribution of anisotropy is reduced so that a temperature dependency of alpha (xcex1) can be reduced. Since an easy axis (that is, axis of easy magnetization) and a hard axis which are factors for magnetic anisotropy are present, with a line which connects a central (111) plane of a stereographic projection chart and a (110) plane on the outermost circumference being a symmetrical axis and, therefore, influences of them are cancelled on the line and reduced adjacent to the line and thus the temperature dependency of alpha (xcex1) can be reduced. Besides the above, the signs of wavelength coefficients are different from each other in the present invention and, therefore, the wavelength dependency can be reduced. If the outer magnetic field is determined to be within the range specified above, the term of alpha (xcex1) is reduced so that the temperature dependency is mainly contributed to by xcex8Fmax. In this case, the temperature dependency of xcex8F can be reduced by reducing the temperature dependency of xcex8Fmax.